Remark 2.1 (Variants) 
For \(p\ge 2\) , the stiffness can be further reduced by imposing least-squares penalties on higher-order derivative jumps. However, these additional terms increase the computational costs while our numerical experiments (not shown for brevity) indicate only a further marginal improvement in terms of spectral errors.
We also mention the recent work [1]} which penalizes both the higher-order derivatives as well as the mass bilinear form near the boundary to eliminate the so-called outliers in isogeometric spectral approximations.

